1. Field of the Invention
This invention relates to the field of data recording, and more specifically to optical recording of data onto a thin thermally sensitive film by means of a modulated laser beam.
2. Description of the Prior Art
It is known that by focusing a laser beam of sufficient intensity onto a thin absorbing film, an area approximately the size of the focused spot can be heated sufficiently to allow a hole to be formed in the film. The beam may be modulated into light pulses representing information to be recorded, which pulses are focused onto a moving medium, commonly a rotating disc. A track of holes is placed in the recording layer, forming a spatial representation of the time modulation. The holes are a result of each pulse carrying sufficient energy to the recording layer to raise the illuminated area above the hole formation temperature. Optical disc recording in this manner has demonstrated a capability to store approximately 10.sup.11 bits of information on a single side of a 12 inch diameter recording disc with a single channel input rate of up to 50 M bit/second. The high performance laser recorders that have been demonstrated are characterized by high peak write power requirements that necessitate gas rather than semiconductor lasers and costly external modulators. This is especially true for multichannel hardware designed to attain data rates in excess of the single channel data rate. In this case the write power requirements necessitate the use of a large water-cooled gas laser. The modulator required is a complex multichannel design involving costly fabrication procedures.
It would be highly desirable to replace the water-cooled gas laser and complex modulator required in a multi-channel recording system with a semiconductor laser array. Semiconductor lasers are small and efficient and would not require water-cooling. They can be directly modulated by modulating their drive current, thus eliminating the need for a separate complex modulator. However, present semiconductor lasers do not produce sufficient power to be capable of high data rate writing while retaining the beam control required for producing the smallest focused spot possible. In addition, operating a semiconductor laser at the power levels necessary for high data rate writing raises the operating temperature of the laser to a point which prohibatively shortens its operating life. This problem would be compounded in a multiple laser array suitable for use as a multichannel writing source. In such an array the power output of the lasers would be limited by the amount of heat that could be dissipated by the small array. A further disadvantage of semiconductor lasers in the prior art apparatus and method is that the recording spot diameter, which determines data packing density, is greater when produced by a semiconductor laser than when produced by the gas lasers typically used as a write source. The longer wavelength associated with semiconductor lasers enlarges the spot diameter to a size which seriously reduces the amount of data which can be stored on an optical disc.